Updated mediawiki/extensions
Project: mediawiki/extensions/Flow  cf0c143ecbed0f90070c9bca14bf3d1b6cbfee09

API: Set 'missing': '', instead of 'missing': 'missing'

The API uses the existence of the key to indicate truthiness.
This shows up if a Flow page does not have a header.

Change-Id: I6b1767183cf223b1f6d3fd41b7c23a862504ec39